1. Field of the Invention
The present invention pertains to the art of vertical cavity surface emitting lasers (VCSEL) and, more particularly, to vertical cavity surface emitting lasers that emit optical radiation having controlled polarization states.
2. Discussion of the Prior Art
It is widely known in the art to use semiconductor lasers in various applications such as image scanning and recognition systems, optical communications and laser disc players. As is well known in the art, a typical semiconductor laser is formed by placing two parallel mirror stacks on either side of an active region to form a laser cavity. Usually a semiconductor P-N or P-I-N junction is formed about the active region. When electrical current is provided across the junction, electrons combine with holes in the active layer to generate radiation which is reflected back and forth between the mirrors. When the applied current is greater than a threshold current of the laser, the optical radiation in the laser cavity coherently oscillates to form a standing wave and, as a result, the laser emits coherent optical radiation. Usually the electrical current is applied through two electrodes, one located adjacent one mirror and the other adjacent the second mirror. Typically, one of these electrodes has some type of hole through which optical radiation may be emitted. In the VCSEL, radiation is emitted in a direction perpendicular to the plane of the substrate, rather than generally parallel to the substrate as in the case of a conventional edge emitting laser.
In the past, typical VCSELs have been manufactured in either a square or circular symmetrical configuration. As a consequence, emitted optical radiation tends to form circular beams which lack a preferred polarization. Essentially each VCSEL cavity has a slight anisotropic optical characteristic which can give a slight preference of polarization in one direction, thus causing the laser to emit a beam of polarized light in a preferred direction. When VCSELs are placed in an array, this tendency can lead to each of the emitted beams having a different polarization. Unfortunately, variations of polarization between different VCSELs within an array can lead to degraded performance which is unacceptable in certain environments, particularly in image scanning and recognition systems. Therefore arrays of VCSELs which have uniform directions of polarization within an entire array are needed.
It has heretofore been proposed by Jewel et al. in U.S. Pat. No. 5,331,654 to use the anisotropy in the atomic, molecular or electronic structure of the materials comprising the VCSEL, create anisotropic features through anisotropic patterning, deliver offset alignment of the features processing of the VCSEL or, finally, form anisotropic structures within the VCSEL, to control the polarization of the emitted beam. Unfortunately, manufacturing VCSELs in this fashion has proven extremely difficult. For example, fabricating an anisotropic structure by etching some type of grating into the semiconductor material is particularly difficult when the width of the etched lines are extremely small as is necessary for the emission of visible or near infra-red wavelengths.
Another solution for controlling the polarization of a vertical cavity surface emitting laser has been proposed by Swirhun et al. in U.S. Pat. No. 5,412,680. Essentially Swirhun et al. teaches that the polarization of a VCSEL laser can be controlled by having an active region containing at least one strained semiconductor layer which has a preferred direction of electrical conductivity due to the strain. In another embodiment the active region is formed in an elongated oval shape which also controls the direction of polarization. In the embodiment related to the increasing of strain in the active region, Swirhun has proposed to use alternating lasers of GaAs and In.sub.0.2 Ga.sub.0.8 As layers. Unfortunately, since the wavelength of the emitted radiation is dependent on the thickness of the In.sub.0.2 Ga.sub.0.8 As, when this layer becomes too thick, discontinuities can arise in these layers due to the excessive strain imposed. In a second embodiment disclosed in the '680 patent in which the active regions are made of oblong shapes, serious costs become involved in trying to obtain the exacting dimensions required of the active region.
Based on the above, there exists a need in the art of VCSELs for an efficient and cost effective method of manufacturing an array of VCSELs whose emitted optical polarization can be easily controlled. In addition, there exists such a need for an attractively manufactured array wherein, all of the VCSELs within the array would have the same polarization of light. This would create an array of polarized laser light which could be used in various fields, particularly imaging and optical communications, that has not been achievable in a simple and economical manner before.